Normally, a process cartridge is detachably mounted in image-forming devices such as laser printers. The process cartridge includes a photosensitive drum functioning to carry an electrostatic latent image, which is developed into a toner image by developer. The photosensitive drum is disposed in confrontation with and in contact with a transfer roller. The toner image carried on the photosensitive drum is transferred onto paper passing through a nip position between the photosensitive drum and the transfer roller by a transfer bias applied to the transfer roller. In this construction, a guide member is typically provided near the photosensitive drum upstream of the nip position in the paper-conveying direction for guiding the paper to the nip position.
Japanese unexamined patent application publication No. HEI-5-53449, for example, discloses a transfer unit provided with the following type of guide member. The guide member includes a paper conveying guide, an eccentric cam for pivoting the paper conveying guide, and a photosensor. When the photosensor detects the leading edge of the paper, the eccentric cam is rotated to place the paper conveying guide near the photosensitive drum. When the photosensor detects the trailing edge of the paper, the eccentric cam is rotated to separate the paper conveying guide from the photosensitive drum.
However, the transfer unit described in Japanese unexamined patent application publication No. HEI-5-53449 requires a photosensor and eccentric cam for placing the downstream portion of the guide member near to the photosensitive drum and separating the same portion from the photosensitive drum. Moreover, the transfer unit requires a motor for rotating the eccentric cam, and a control unit for driving the motor at a prescribed timing based on signals received from the photosensor, thereby requiring a larger number of parts and a more complex construction. This construction may also be less reliable because electronic units, such as the control circuit board on which the photosensor and control unit are provided, are susceptible to electrical malfunctions.